The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent the work is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
A dimmer for an incandescent lamp may include an input circuit with a thyristor. The thyristor switches ON at an adjustable time (or phase angle) subsequent to a start of each alternating current (AC) half-cycle of a power signal received from an AC power source. Switching ON and OFF the thyristor affects a voltage waveform received at the incandescent lamp. The input circuit controls current supplied to the incandescent lamp based on a control input. The control input may be changed to adjust a dimming level of the incandescent lamp.
A thyristor operates as a bistable switch conducting when a current trigger is received and continuing to conduct while in a forward biased state. Since the thyristor switches ON and OFF instead of absorbing a portion of the power supplied from the AC power source, the amount of wasted power due to the thyristor is minimal. This is unlike traditional variable resistors, which absorb (or convert) a portion of the AC power provided to thermal energy.
As an alternative to using a single thyristor, a dimmer may include a TRIAC. A TRIAC includes two thyristors, operates as a bidirectional switch, and has specific latching properties. In order to latch a TRIAC in an ON state, a trigger voltage is applied across terminals of the TRIAC. The trigger voltage is greater than a predetermined voltage level to enable a minimum current (or latch-up current) to flow through and latch the TRIAC in the ON state. The latch-up current may be, for example, several milli-amps (mA) to tens of mA. The latch-up current must flow for a certain period of time to completely latch the TRIAC in the ON state. Subsequent to latching the TRIAC, a predetermined level of current (or hold-up current) is supplied to maintain the TRIAC in the ON state. The hold-up current may be less than the latch-up current. Different TRIACs have different latch-up and hold-up current levels.
Solid-state lighting refers to providing light from solid-state lamps, such as light from light emitting diodes (LEDs). A solid-state lamp can provide the same level of luminous intensity as an incandescent lamp while consuming less power. As an example, a LED may consume 6 watts of power for a given luminous intensity, whereas an incandescent light may consume 60 watts of power for the same luminous intensity. The luminous intensity per power usage rating (e.g., luminance/watt rating) of solid-state lamps continues to increase with technology improvements in solid-state lamps. As a result, solid-state lamps can provide a high level of luminous intensity (at least the same as an incandescent lamp) while operating based on a low level of power and/or current (less than that used by an incandescent lamp).
Due to the low power operation and corresponding operating current levels of solid-state lamps, it can be difficult to maintain a hold-up current using traditional TRIAC dimmers. The smaller the current level, the more difficult it is to maintain a sustainable hold-up current. A traditional TRIAC dimmer may be configured for an incandescent lamp having a full power rating of, for example, 60 W. Since a solid-state lamp may have a full power rating of, for example, 6 W during deep dimming (or low luminous intensity output) conditions, operating power may be less than 1 W. As a result, the current through a TRIAC of a traditional dimmer may decrease to a level less than a hold-up current and the TRIAC may shut OFF. Thus, load applications and dimming performance is limited for traditional TRIAC dimmers.